One method of producing fine powder from polyethylene is described in German Auslegeschrift No. 1 229 709. This method is based on the treatment of solids to form granules, sheets and other forms of small particles in which the solid is pulverised, in a dry state, by comminution. By introducing a so-called crushing pressure in a roller gap, the solids are comminuted. However, it is often necessary to carry out four to six passes of the material through a roller gap provided with heated rollers to achieve satisfactory comminution. Subsequently, the material is comminuted in a mill which has also been heated to approximately the same temperature as the heated rollers, that is to say, between 75.degree. C. and 80.degree. C.
Such prior specification also describes the chemicophysical phenomenon that the molecular bonding of the polyethylene chains are loosened by use of a crushing pressure in a roller gap in a predetermined temperature range. This phenomenon makes it possible to break down polyethylene and to convert it into a fine powder. However, this method has not proved successful in practice. Firstly, the amount of energy required to comminute a solid material is considerable, particularly if the material has a high tensile strength. In addition, it is very energy-consuming, and therefore uneconomic, to heat solid polyethylene to a temperature of between 70.degree. C. and 80.degree. C. (as described in Example 1 of German Auslegeschrift No. 1229709) or even to a temperature of between 100.degree. C. and 105.degree. C. (Example 2 of such specification) both when heat is introduced by thermal conduction from externally to the interior of the solid particles, and when the heat is introduced by friction heating in the roller gap. A relatively long period of time is required in both cases.
In short, therefore, it may be stated that this fine grinding method is uneconomic because of the high energy consumption necessary to pulverise a solid and to heat the solid to a suitable temperature, the number of different types of machines needed, the discontinuity of the method due to the necessity of effecting a number of repeated passes through both a rolling mill and a grinding mill and the time-consuming, and thus industrially inefficient, nature of the method.
However, German Auslegeschrift No. 1 090 422 describes a method of pulverising or fine-grinding polyolefins which are in the form of a fused mass. By way of example, polyethylene is fused in a discontinuous kneading device which operates with a stamping or punch pressure. The wall of the kneading device and the kneading blades themselves are tempered with superheated steam at a temperature of up to 200.degree. C.
The fused mass is then rapidly cooled to a temperature of not more than 10.degree. C. Simultaneously, the speed of the kneading blades and the stamping or punch pressure are each reduced by approximately 50%.
This method is, however, also discontinuous and suffers from certain disadvantages which make it unsuitable for industrial use.
In this respect, it is firstly necessary to point out that large amounts of energy are wasted when a molten polyethylene mass is quenched from 200.degree. C. to 10.degree. C. This is chiefly due to extremely poor thermal conductivity of polyethylene. It will also be evident that cooling of this nature takes time which is a further disadvantage. These two disadvantages alone render the industrial utilisation of such method unsuitable.
However, in addition, the reduction of the speed of rotation of the kneading blades slows down the pulverising process to a disadvantageous rate and the mechanical equipment cannot, therefore, be operated continuously.
On the other hand, there has been a long-felt desire for obtaining polymers in powder form, because powder coatings on, for example, pipes to protect the pipes against corrosion are very much in demand. However, a powder having a uniform grain or particle size (DIN 55 990) is necessary on an industrial scale to enable a wide range of powder lacquers to be produced. Hitherto, however, the obtaining of such a powder has only been possible using a complicated process involving the use of a large number of separate machines.
In "Kunststoffe" (Volume 64 (1974), Book 2, Pages 54 to 57), a method is described for producing coating powders from plastics materials. FIG. 1 on Page 55 of this journal illustrates an apparatus used for carrying out such a method, the apparatus including a preliminary mixer E, a metering device F, a double-shafted screw kneading device G having a discharge housing H, two cooling rollers I, a cooling drum K, an additional pair of crushing rollers M and an additonal pulveriser O.
The polymer is fused in the double-shafted screw kneading device and is then transferred to a rolling mill by means of a wide-mounted nozzle. In the rolling mill, a crushing pressure is exerted on the material. The large cooling roller which is disposed downstream thereof provides additional cooling. The cooled material is then conveyed to a preliminary comminution device to produce substantially chip-like particles which are subsequently conveyed into the grinding and screening apparatus.
It is readily apparent that such a method is complicated and laborious and must, therefore, be uneconomic.